In recent years, considerable efforts have been made by the coatings industry to develop coating formulations containing little or no volatile organic compound (VOC) content. Regulations to limit the amount of VOC content of industrial coatings have encouraged research and development to explore new technologies directed at reducing solvent emissions from industrial solvent-based coatings operations used to coat such products as automotive parts, appliances, general metal products, furniture, and the like. However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC content coating compositions must still meet or exceed the performance standards expected from solvent-based compositions.
Alkyd resins are one of the most common binders used for ambient-cure, solvent-based coatings. The resistance properties of traditional solvent-borne alkyd resins are developed via autooxidative crosslinking of the alkyd film. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidized in air to give hydroperoxides which subsequently decompose to generate free radicals, resulting in oxidative crosslinking. This oxidative crosslinking process is commonly accelerated by adding driers, such as, for example, various salts of cobalt, zirconium, calcium, and manganese. However, while alkyd resins have shown, and continue to show, promise, they have relatively slow “dry” and/or cure times, particularly at ambient temperatures. Various modifications have been made to alkyd resins to address such concerns.
One such attempt includes polymerization of an alkyd resin with a vinyl compound, such as styrene or methyl methacrylate, via a free-radical reaction, to produce a vinyl-alkyd copolymer or a vinyl alkyd. Vinyl alkyd resins generally have a higher molecular weight and a higher Tg, producing coatings with reduced tack-free time (solvent evaporation). However, the through-dry time (oxidation of the film) of such coatings is longer due to the decreased degree of unsaturation in the alkyd as a result of copolymerization with the vinyl compound. This problem is described in further detail in Resins for Surface Coatings, Vol. 1, pp. 181 et seq., ed. by P. K. T. Oldring and G. Hayward, SITA Technology, London, UK, 1987, which is incorporated herein by reference. An additional drawback is that paint formulations containing vinyl alkyd resins require greater amounts of solvent, due to the increased molecular weight and Tg of the vinyl alkyd.
JP 48085628 (hereinafter JP '628) describes light-curable coating compositions made from a drying oil-modified alkyd resin which is further modified using glycidyl acrylate, glycidyl methacrylate, or its derivative. In this reference, drying oil-modified alkyd resins having —CO2H groups and an oil length of 20-80 are treated with glycidyl acrylate, glycidyl methacrylate, or its derivative, in the presence of a polymerization inhibitor. In a specific embodiment, a drying oil-modified alkyd resin having an acid number of 100 and an oil length of 34 is reacted with 36 parts glycidyl methacrylate, to give a resin varnish having an acid number of 5.0.
Both the acid number of the drying oil-modified alkyd resin of JP '628, and the amount of glycidyl methacrylate used, are relatively high, requiring the use in the reaction mixture of hydroquinone, a polymerization inhibitor, to prevent the alkyd from gelling during resin synthesis. One drawback of this approach is that the presence of a polymerization inhibitor in paint formulations is known to prolong the drying times of the resulting coating films. Moreover, the disclosed alkyd resin composition of JP '628 contains an amine monomer, triethanolamine, which is desirable for the UV cure application intended, but can cause detrimental effects on oxidative cure. The resin in JP '628 is afterward mixed with a photosensitizer or a photoinitiator to give a coating composition which hardens with UV irradiation. Accordingly, the disclosed coating composition requires the use of a photosensitizer or photoinitiator, and UV irradiation, in order to carry out the teaching. The reference does not teach a coating composition suitable for ambient oxidative cure, high-solids coating applications.
Thus there still exists a need in the art for a modified or functionalized alkyd resin capable of undergoing crosslinking upon film formation, which can be used to prepare ambient oxidative cure, fast-dry, and high solids coatings, having low VOC content. Such coatings would ideally exhibit the properties and advantages of high VOC content coatings.